The present invention relates generally to an ignition interference suppressor method and device for cars. Ignition circuits for internal combustion motors comprise mainly a low voltage circuit 1 and a high voltage circuit 2 (FIG. 1). The low voltage circuit comprises a direct current source, the low voltage winding 4 of a coil 3 and breaker points, and the high voltage circuit comprises the high voltage winding 5 of the coil feeding spark plugs, through a distributor 6. Such circuit layouts have the drawback of radiating interference which acts as a nuisance to high frequency waves reception in the surroundings. For these reasons, various anti-parasitic devices have been proposed. It is known to use in the high voltage ignition circuit of internal combustion engines distributed or lumped resistances (e.g. in the spark-plugs, plug caps, in the distributor, the connections or the ignition cables), which function essentially to reduce the power of the spark (which is the interference source), and to reduce the antenna radiation efficiency of adjacent parts, and in this way operate as an antiparasitic or anti-interference device.
In a typical application, an ohmic resistor of 15 k.OMEGA. is placed in the spark-plug, and in addition the ignition cable has a linear resistance of 15 K.OMEGA.. Resistance values of 10 K.OMEGA.to 100 K.OMEGA. are currently used in the secondary circuit of the coil.
With a high voltage generating system of sufficient power and voltage to take in account the thereby lost ignition energy, correct operation of the engine is nevertheless secured.
But, evidently, the use of such resistances is inefficient from both a technical and an economical viewpoint. Expensive high voltage ignition power is produced for thereafter having a part of it destroyed: low reliability and short service life semi-conductive materials are used, and special spark plugs (with built-in resistor) which cost is two or three times that of usual spark plugs, in view of suppressing interferences. There is no doubt that such a solution is not cost-effective. Modern methods for the suppression of interferences in ignition devices involve a frequency-selective absorption: the high voltage circuit has a low DC-resistance (e.g. 100 .OMEGA. to several K.OMEGA.) for maintaining the power of the spark due to the direct current and the low frequency components of the ignition current; but it has a very high resistance for the high frequencies, which represent the interference. Such devices are disclosed in U.S. Pat. Nos. 3,191,132--3,309,633 and 3,573,676.
For the high frequencies, the antiparasitic effect of such frequency-selective cables is no longer limited, since in this range such absorption effect can be unlimited without even impeding ignition. Besides, the metallic conductor in the cable has a high grade reliability. Due to these facts, such ignition cables have been broadly used these last years in many countries.
But, if such cables and plug caps are used in a car designed for high resistance suppressor elements in the ignition circuit, the following drawbacks occur
(1) the ignition current is too high, which can increase the wear of the spark plug;
(2) the increased ignition current can cause more intense secondary parasitic radiations to be radiated either by inductive or capacitive coupling through various parts (acting as secondary antennas) placed in the proximity of the radiation source, such as spark plug heads, distributor's heads, etc, or by increased direct radiation of these parts. Due to the stronger spark, there results at least a partial deterioration of the high quality overall suppression effect of the cables with frequency-sensitive resistance.
It is known, particularly, that this secondary radiation presently sets the essential limitations of these improved antiparasitic methods and devices, more especially in the AM-radio frequency range (where frequency-selective absorption is not efficient) and in the case where very low RFI-fields are desired.
It is possible, of course, to shield radiating parts (to put shielded caps on spark plugs and on, or in, the distributor, etc . . . ) but this is expensive and may have other drawbacks.